1. Field of the Invention
This invention relates to a process for coloring expandable polystyrene beads. More particularly, it relates to a process for coloring, at a high efficiency of dye penetration, expandable polystyrene beads without any addition of a blowing agent which increases the pressure of the gaseous phase for the prevention of foaming of said beads in the process of coloring or a solvent which serves as a dye penetration aid, said colored beads forming molded foams of satisfactory coloration and an excellent appearance.
2. Description of the Prior Art
Polystyrene beads impregnated with a blowing agent have been extensively used for the manufacture of molded foams, and colored expandable beads have been used for the manufacture of colored foams.
The following methods are known for coloring expandable beads: (A) adding a dye or a pigment to expandable beads in a vessel and agitating the mixture; (B) suspending expandable beads in water and adding to this aqueous suspension a dye and a blowing agent or a solvent; and (C) adding a dye to an aqueous suspension of resinous beads in the polymerizing step of the resin, thus coloring the beads simulaneously with impregnation with a blowing agent.
The method (A) is usually adopted in expansion molding factories. However, a dye or pigment applied by this method does not stick fast to the surfaces of expandable beads, hence, it tends to be washed off by steam used in a preexpansion or a molding step. Particularly in a molding step, steam is violently ejected through slits of a mold, making the tint around the slits lighter than at areas other than at the slits. It is therefore difficult to obtain molded foams uniformly colored. In addition, the distribution of the dye or pigment on the surfaces of the expandable beads is not even, resulting in a nonuniform coloring. There is also some difference in coloration between the surface and the inside of the bead, as can be seen when an expanded bead is split. Furthermore, this method is normally carried out by agitating the mixture in an open vessel for 20 to 30 minutes, thus permitting the blowing agent contained in the beads to be released and hence resulting in lowered expandability.
Also in the methods (B) and (C), a difference in coloration between the surface and the inside of an expanded bead is unavoidable, because most colorants commonly used are very slow to penetrate into the bead and hence tend to remain near its surface.
Foams molded from expandable beads colored by any of the methods (A), (B) or (C) also have the problem that colorant migration often takes place to contaminate other objects because of low adhesion of the colorant.
In the method (B), a technique is also adopted in which the aqueous suspension is heated to a temperature higher than the softening point of the expandable beads to accelerate dye penetration. In this case, heating to such a temperature without foaming of the expandable beads requires addition of a blowing agent to increase the pressure of the gaseous phase in the system. The amount of the blowing agent added to the system needs to be enough for saturating the gaseous phase at a coloring temperature. Meanwhile, most of the blowing agent added at an ambient temperature in the form of liquid is consumed to penetrate into the expandable beads, and hence it is necessary to add the agent in an amount greater than needed theoretically to prevent foaming of the beads, resulting in an increase in the content of the blowing agent in the beads, even when it is not intended. When general-purpose expandable beads of a specified high content of a blowing agent, for example, are colored by the method of (B) immediately after production of the beads, the colored beads will contain a larger amount of the blowing agent, adversely affecting expandability and moldability of the beads and the cell structures of molded foams. It is therefore necessary to use expandable beads produced especially for coloring whose blowing agent content is lower than specifications, or to pretreat general-purpose beads before coloring to lower the content of the blowing agent by a suitable method. This is unfavorable to an industrial application.
To solve the problems described above, we tried to color expandable beads at a temperature higher than the softening point thereof while preventing foaming of the beads by introduction of an inert gas such as nitrogen to increase the pressure in the gaseous phase. But no satisfactory result was obtained.
Our studies also revealed that heating expandable beads to allow a blowing agent to move rapidly in the beads adversely affects moldability of the beads and their cell structure, physical properties and the appearance of molded foams, and that similar troubles also occur when expandable beads are further impregnated with a blowing agent in the form of a liquid.